Recently, a variety of golf balls have been proposed that aim to achieve certain intended spin properties and increase the distance traveled by the ball, both by providing the ball with a multilayer structure and also by imparting the core that makes up most of the ball with a specific hardness profile in such a way as to optimize the core hardness profile and the overall hardness and thickness parameters of the ball. Art which provides the core of a golf ball with a two-layer structure and satisfies a desired core hardness profile has hitherto been proposed. Golf balls in which the core is made of two layers are described in, for example, U.S. Pat. Nos. 7,115,049, 7,267,621, 7,503,855 and 8,702,535.
However, none of these patent publications specify the relationship between the deflections of the inner core layer and the outer core layer when subjected to specific loading, nor have the core hardness profiles in any of these disclosures been fully optimized. Hence, mid- to high-level golfers and professionals with intermediate to high head speeds have awaited the emergence of new golf balls containing a two-layer core that can achieve an even longer distance and that are also able to maintain the spin performance on approach shots at a high level, thus further increasing the enjoyability of the game.